This invention generally concerns high-efficiency methods of grinding ceramic workpieces, and is specifically concerned with a method for grinding zirconia workpieces with a silicon carbide grinding wheel at a high infeed rate.
Methods for shaping and machining ceramic workpieces with grinding wheels are well known in the prior art. The workpieces may be, for example, the zirconia plungers used in diesel engine fuel injectors. The transition-toughened zirconia used to form such plungers has a Knoop hardness of between about 1,000-1100 kg/mm.sup.2. In the past, grinding wheels employing either diamond or CBN (carbon-boron-nitrogen) abrasives have been used having Knoop hardnesses of 7,000 kg/mm.sup.2 and 4,800 kg/mm.sup.2, respectively. While the relative hardness of diamond and CBN abrasives allows such grinding wheels to effectively shape the softer zirconia blanks into fuel injector plungers, such abrasive materials are very expensive. Less expensive abrasive materials are known which are still considerably harder than transformation-toughened zirconia. For example, silicon carbide in a green state has a Knoop hardness on the order of 2,800 kg/mm.sup.2, which is considerably higher than the Knoop hardness of 1,000-1,100 kg/mm.sup.2, associated with zirconia. Unfortunately, attempts to use less expensive silicon carbide grinding wheels to machine zirconia and ceramics of like hardnesses have not yet met with any practical success. But before the meaning of the term "practical success" can be understood in this context, some additional background information is necessary.
In order for a grinding operation to be efficient and effective, at least three factors must be present. First, the ratio of the volume of material removed from the workpiece must be substantially higher than the volume of material worn away from the grinding wheel as a result of the grinding operation. This factor is known as the G-ratio. It is a parameter used extensively to characterize the effectiveness of a grinding wheel for a specific work-material under a given setup. A high G-ratio means the grinding wheel will have less wear to remove a specific volume of work-material and better control of the cut tolerances. Due to the uneven wear in the grinding wheel, the G-ratio is frequently calculated on the basis of the average diametral wheel wear, .delta..sub.avg, which may be expressed as follows: ##EQU1##
where
d.sub.1 and d.sub.2 are the diameters of the front and back ends of the ground workpiece, and PA1 d.sub.3 and d.sub.6 are the diameters of the grinding wheel across different sections, as measured in a plastic molding made of the worn wheel. PA1 N is the number of ceramic parts ground, PA1 D is the initial diameter of the ceramic blank, and PA1 D.sub.w is the diameter of the grinding wheel.
This factor may then be used to calculate a G-ratio designed as G.sub.avg as follows: ##EQU2##
where
A G-ratio of 1 would indicate that the volume of material removed from the grinding wheel as a result of wheel wear was the same as the volume of material removed from the workpiece. Such a low ratio is generally unacceptable, since it indicates that the grinding wheel would have to be retrued after only a few workpieces had been ground. Such frequent grinding wheel reshaping is not only expensive, but also time consuming. Generally speaking, the G-ratio must be on the order of about 5 or higher for an acceptable degree of economy to be realized in production grinding.
A second required factor is that the grinding operation must accurately machine the workpiece to within the required tolerances. For example, if the purpose of the grinding operation is to machine a piston head around a blank ceramic workpiece, then the circular cross section of the piston head must conform to a high degree of roundness, or the piston head will either not fit into its cylinder bore during assembly, or will fail to generate adequate compression within the bore. This particular factor may be expressed as "roundness", and is expressed in terms of the maximum linear distance variation between measured roundness and true roundness. For example, a roundness of 0.01 mm would indicate a maximum variation from true roundness of 0.01 mm along all diameters.
The third required factor is surface finish, which is an indication of the roughness of the resulting ground surface on the ceramic workpiece. In the U.S., surface finish is usually expressed as the arithmetic average of variations in the surface from planarity, and is designated as Ra.
There are other factors that can be considered when evaluating the efficiency and effectiveness of a grinding operation, but G-ratio, roundness and surface finish are certainly among the most important in a manufacturing operation as they bear directly on wheel wear and the resulting quality of the machining operation.
Previous attempts to grind zirconia workpieces with relatively inexpensive silicon carbide grinding wheels have failed to produce high-tolerance cuts within acceptable G-ratios. The G-ratios associated with such attempts almost never been higher than 2.0, and are more typically 1.0 or less. Worse yet, the lack of accuracy of the cuts made in such prior art grinding operations has precluded the use of such low cost grinding wheels where tight tolerances are required. The frequent wheel retruing and replacement associated with such low G-ratios, in combination with the inaccurate cuts made by such wheels has resulted in the near exclusive use of diamond or CBN-type grinding wheels for the precision machining of zirconia ceramic components, despite their high cost.
Clearly, there is a need for a method of producing high-tolerance cuts in ceramic materials such as transformation-toughened zirconia and silicon nitride without the use of expensive diamond or CBN grinding wheels. Ideally, such a method would employ silicon carbide grinding wheels which could perform a high-tolerance cut in ceramic workpiece with high G-ratio and superior surface finish.